In a rotary machine such as a steam turbine and a gas turbine, there is used a non-contact type seal structure such as a labyrinth seal, in order to prevent leakage of a working fluid such as steam from a gap formed between the stationary side and rotor side.
As a labyrinth seal there is known a step-type labyrinth seal in which, on an inner circumference of a casing that serves as an outer shell of a rotary machine, there are provided seal members such as seal fins that extend toward rotor blades, and there is provided a step-shaped shroud provided on the tip end of the rotor blades (for example, refer to Patent Document 1).
As shown in FIG. 7, a step-type labyrinth seal 102 includes; a plurality of seal fins 17, 18, and 19 that extend from a casing 10, and a step part 3 formed on a shroud 51 which is provided on the tip end part of a rotor blade 50. The labyrinth seal 102 is configured of an upstream side cavity 25 that is formed by a front-facing step, and a downstream side cavity 26 that is formed by a rear-facing step.
Within the upstream side cavity 25 formed by a front-facing step, a leak jet SL which has traveled through an upstream gap mA between an upstream seal fin 17 and a base surface 4 of the shroud 51, forms a vortex B while colliding with an upstream side surface of the step part 3, thereby deflecting the leak jet SL. As a result, blow-by of a leak jet SL2 into an intermediate gap mB between an intermediate seal fin 18 and the step part 3 is suppressed and the amount of leakage is reduced.